The present invention relates to a magnetoresistive random access memory (MRAM).
A magnetoresistive random access memory (MRAM) is expected to be a next-generation high performance memory which will substitute for DRAM and flash memory because of high speed, low power consumption, extremely high non-volatility, and rewrite resistance exhibited thereby, so that many foreign and national companies have studied and developed MRAM for practical use. The MRAM is generally a memory which makes use of the tunnel magneto-resistance (TMR) effect, and since the TMR ratio must be improved for accomplishing higher speed and higher reliability, investigations are extensively under progress for the material composition, deposition method, and the like which could result in a higher TMR ratio.
A TMR element as illustrated in FIG. 1 is used for a memory unit of MRAM. This element has a structure of sandwiching an insulating layer, called a “tunnel barrier layer,” between ferromagnetic layers. As a voltage is applied to this element in the interlayer direction, a current flows due to the tunneling effect, wherein the value of the electric resistance differs depending on the magnetization directions of the two ferromagnetic layers. Generally, the resistance is smaller when the magnetization directions of the two ferromagnetic layers are parallel than when they are antiparallel. The MRAM utilizes this difference in resistance as bit information. Also, the ratio of the differences in resistance when the magnetization directions of the two ferromagnetic layers are parallel to when they are antiparallel is called the “TMR ratio.” Assuming that the spin of electron is preserved during tunneling, the TMR ratio (R) is expressed by the following equation (M. Julliere: Physics Letters 54A, 225 (1975)):R=2P1·P2/(1−P1·P2)  (1)where P1, P2 are spin polarizabilities of the respective ferromagnetic layers (0<P1, P2<1). Thus, the polarizabilities P1, P2 should be increased in order to achieve a high TMR ratio. While sufficient understanding has not been acquired at present about factors which determines the polarizability P, the use of a magnetic material having larger magnetization (or magnetic moment) is deemed as a guideline for achieving a higher TMR ratio (Kouichiro Inomata, Applied Physics 69, 186 (2000), Japanese Society of Applied Physics).
On the other hand, the magnetic film may be made of Fe, Co, Ni or an alloy thereof, and it has been theoretically predicted that the magnetic moments of these materials vary due to strain applied thereto (Cerny et al., Physical Review B67, 035116 (2003)).